Process for autoelectrolytic reproduction of documents



United States Patent 3,477,847 PROCESS FOR AUTOELECTROLYTIC REPRGDUCTIONOF DOCUMENTS Georges Paul Marie Becquerel, Vincennes, France, as-

signor to Eastman Kodak Company, Rochester, N.Y., a corporation of NewJersey No Drawing. Filed July 16, 1965, Ser. No. 472,699

Int. Cl. G03g 5/04, 7/00 US. Cl. 96-1.8 5 Claims ABSTRACT OF THEDISCLOSURE A photosensitive autoelectrolytic element. is disclosed whichincludes a backing sheet coated with a mixture of photo-conductiveinsulating material and a metal having negative electrode potential withrespect to hydrogen dis persed in an insulating film formingmaterialpThe element is exposed to an image and developed by contactwith a solution containing a reducible solute.

The photocon process employs a photosensitive element which comprises aphotoconductive, electrically insulating coating on an electricallyconductive backing. A preferred photoconductive insulating layer forphotocon comprises photoconductive zinc oxide dispersed in an insulatingresin vehicle. By exposure of this insulating coating, to a lightpattern, there is formed a conductive image on photoexposed surfaceareas on the coating, while unexposed areas remain electricallyinsulating. An image is then developed on the exposed areas byelectrodeposition. A solution of an electrolytic salt, e.g. a silversalt solution, is contacted with the surface of the exposedphotoconductive coating while voltage is applied across thephotoconductive coating, between the conductive backing as a cathode andthe electrolytic solution which contacts the anode and thephotoconductive surface. At photoexposed areas the coating is conductiveand current across the photoconductive coating causes reduction andelectrodeposition of the cation from electrolyte solution. At unexposedareas of the coating there is no current, hence no electrodeposition.

Similarly, the process of the present invention employs anelectrophotographic element having a coating that comprisesphotoconductive particles dispersed in an insulating vehicle on abacking support, and employs an electrolytic solution for developmentafter photoexposure. But the photosensitive element of the presentinvention is autoelectrolytic, that is, when an electrolyte is appliedthere is required no externally applied voltage across thephotoconductive insulating layer to cause electrolysis.

The novel photosensitive element of my invention comprises aphotosensitive autoelectrolytic coating which "ice comprises a mixtureof photoconductive particles and metal particles dispersed in aninsulating vehicle, usually an insulating resin binder.

The novel process of my invention comprises the steps of the firstexposing to actinic radiation selected areas on the photosensitiveautoelectrolytic element and then applying to the surface of the exposedcoating an electrolyte solution containing a solute that will form avisible image by reduction at cathode sites on the photoexposed areas.

Following is a detailed description of a specific example showing apreferred embodiment of the invention.

EXAMPLE I (A) Preparation of the photosensitive element To a solution of4 grams Pilolite A.C. resin in 15 ml. toluene, we add 30 grams ofphotoconductive zinc oxide and 5 grams of fine zinc filing (25 micronsdiameter), with thorough mixing. We add 100 ml. of toluene and ball millthe mixture for 24 hours. We coat the ball-milled mixture on a paperbacking support and dry, first in air then in an oven at C. for severalhours to remove residual solvent. The finished element is dark-adaptedfor several hours prior to photoexposure.

(B) Exposure and development The coated element is exposed at the coatedsurface to an actinic light pattern from a watt mercury vapor lamp at adistance of 30 cm. for 8-10 seconds. A developable latent image persistsfor about two minutes and during this time the exposed element isdeveloped by applying an electrolytic silver solution to the exposedsurface. A preferred electrolytic solution consists of Water to make 100ml.

To obtain more even development we prefer to prewet the surface bysoftly rubbing water on the coating just before applying the developersolution. After applying the developer, a silver image develops at theexposed areas in about 510 seconds. The element may then be washed withwater to remove residual developer and dried. No further fixing ordesensitization is necessary.

From the preceding example, it will be seen that the novelphotosensitive element and the novel process of the present inventionprovide a rapid photocopy system of excellent reliability employingrelatively inexpensive materials. There is no need in the photosensitiveelement for a conductive backing, nor is there need in the process foran applied voltage across the photosensitive layer, as are necessary inthe photocon process. The novel process is especially adaptable to usein document copying. The results are comparable to those obtained by thephotocon process.

The electrochemical phenomenon by which the process of the inventionproduces a reduced silver image is not entirely understood. Experimentslead us to think the electrolysis proceeds as follows. When a. zincparticle in the insulating coating is contacted with electrolyticsolution, zinc ions are spontaneously dissolved, leaving negative chargeon the zinc particle. In areas made conductive by photoexposure, excesselectrons are conducted from the zinc particle to an adjacentphotoconductive zinc oxide particle also in contact with theelectrolyte. This conductive zinc oxide particle in turn acts as acathode providing a reduction site for silver ion. As reduction occursat the cathode, zinc continues to be dissolved, generates additionalions, and electrolytic deposition proceeds.

My invention is not limited to the preferred example described above. Inplace of photoconductive zinc oxide in the coating may be substitutedother photoconductive insulating particles, for example, photoconductivestannic oxide, cadmium sulfide and the like. Instead of zinc powder Imay substitute other metals having negative electrode potential withrespect to hydrogen. For example I may use aluminum or magnesium powder.I prefer zinc powder because of its better resistance to surfaceoxidation which may inhibit the electrolytic activity of the metalparticles in the process. Instead of reducible silver ion in theelectrolyte I may substitute another reducible cation that is capablewhen reduced of forming a visible image at the reduction site, forexample, silver, gold, tetrazolium, copper, mercury and the like. Weprefer to use a complex formed by the metal with a complexing agent.Examples of suitable aqueous electrolytes are 1% KAuCl 1% KAuCl +2%thiourea; 0.5% CuSO -5H O; 1% Na S O -5H O.+0.5% Na SO +0.35% AgNO 0.12%AgNO +0.1% mercaptoethylamine-HCI, 1% 2,3,5 triphenyl 2H tetrazoliumchloride; 0.5% N-methyl-2,4-dinitrobenzyl pyridinium paratoluenesulfonate; 1% molybdic acid, and the like. Also, instead of a reduciblesilver in the electrolyte I may use a pH-sensitive dye that will form avisible colored precipitate in basic solution. At the photoconductivecathode, electrolysis liberates hydroxyl ion which will reduce andprecipitate such dye in the immediate vicinity of the cathode. Thus,reducible dye formers may be employed in the electrolyte instead ofreducible silver ion to form a visible image at the photoconductive zincoxide cathode sites. Suitable indicator dyes for this use include CottonBlue BB, Aluminum Red B, Lusanne Brilliant Blue B and Methylene BlueChloride and the like.

To extend the spectral sensitivity of the photosensitive compound I mayadd sensitizing dyes in minute quantities suitable for the sensitizingfunction. The sensitizing dyes may be coated on the photosensitivecompound before it is mixed with other coating ingredients or it may beadded directly in the coating mixture prior to coating. I may use, forexample, Rose Bengal, Fluoroscein, Crystal Violet, and the like assensitizers to extend spectral sensitivity of the photoconductor. Anumber of other spectral sensitizing dyes for photoconductive zinc oxideare described in the literature, for example in US. Patent No.3,052,540, patented September 4, 1962, by H. G. Greig.

A preferred binder is Pliolite S7 (a styrene copolymer-Goodyear Tire &Rubber Co., USA.) but other suitable insulating vehicles include 87%vinyl chloride- 13% vinylacetate copolyrner, Styressol 4444 (aStyrenated alkyd resin, Reichold Chemicals, Inc., U.S.A.), and otherinsulating, film-forming resins.

The ratio of photoconductive particles to binder in the photosensitivecoating is found to influence photographic speed and image density.Increased photoconductor concentration tends to increase speed anddensity but also tends to increase background development (fog).Operable coatings may contain from about 3 to about parts by weight zincoxide to one part binder. With Pliolite 5-7 we find the optimum ratio ofzinc oxide to binder is about 7 to l. The optimum ratio by weight ofzinc oxide to zinc metal in the coatings will depend to some extent onthe zinc particle size, purity of the zinc metal and other factors. Weprefer to use zinc particles of about 3-5 micron size at a ratio of 1part by weight zinc to about 6 parts zinc oxide but we may use 1 partzinc to every 3-10 parts zinc oxide.

In addition to zinc or other equivalent metal powder and zinc oxide orother equivalent photoconductor in the coating, I am also include ironparticles. We once thought iron particles were necessary to provide acathode for the cell but found this is not so.

The backing support for the photosensitive coating may be, but need notbe, electrically conductive and we may use any suitable material such aspaper, metal foil, resin film, and the like for a support. We prefer apaper support for most purposes. We prefer to apply the coating to thebacking by conventional solvent coating methods but extrusion coating orhot melt coating methods can also be adapted for making photosensitiveelements of my invention.

It will be understood that modifications and variations may be madewithin the scope of the invention as described above and as defined inthe following claims.

I claim:

1. A photosensitive autoelectrolytic element comprising a backingsupport and coated thereon a photosensitive layer comprising a physicalmixture of discrete particles of photoconductive insulating material anddiscrete particles of a metal having negative electrode potential withrespect to hydrogen dispersed in an insulating film-forming vehicle,there being 3 to 10 parts by weight of photoconductive insulatingmaterial for each part by weight of metal and about 3 to about 10 partsby weight of photoconductive insulating material for each part offilmforming vehicle, said layer having the capability after exposure toactinic radiation of spontaneously inducing voltage in an electrolytesufiicient to cause electrolytic reduction at cathode sites inphotoexposed areas of said layer.

2. A photosensitive autoelectrolytic element comprising a backingsupport and coated thereon a photosensitive layer comprising a physicalmixture of discrete particles of photoconductive insulating zinc oxideand discrete particles of zinc metal dispersed in an insulating resinfilm, there being 3 to '10 parts by weight of zinc oxide for each partby weight of zinc and about 3 to about 10 parts by weight ofphotoconductive insulating material for each part of film-formingvehicle, said layer having the capability after exposure to actinicradiation of spontaneously inducing voltage in an electrolyte sufficientto cause electrolytic reduction at cathode sites in photoexposed areasof said layer.

3. A photosensitive autoelectrolytic element comprising a backingsupport and coated thereon a photosensitive layer consisting essentiallyof a physical mixture of photoconductive insulating particles and metalparticles dispersed in an insulating film there being 3 to 10 parts byweight of photoconductive insulating particles for each part by weightof metal particles, and about 3 to about 10 parts by weight ofphotoconductive insulating materialfor each part of film-formingvehicle, said layer having the capability after exposure to actinicradiation of spontaneously inducing electric voltage in an electrolytesufficient to cause electrolytic reduction at cathode sites inphotoexposed areas of said layer.

4. An electrophotographic process employing a photosensitiveautoelectrolytic element, which element comprises a photosensitive layercomprising a physical mixture of photoconductive insulating particlesand metal particles dispersed in an insulating film, there being 3 to 10parts by Weight of photoconductive insulating particles for each part byweight of metal particles, and about 3 to about 10 parts by weight ofphotoconductive insulating material for each part of film-formingvehicle, said layer having the capability, after exposure to actinicradiation, of spontaneously inducing voltage in an electrolytesufiicient to cause electrolytic reduction at cathode sites inphotoexposed areas of said layer, said process comprising: (1) exposingselected areas on said photosensitive layer to acitnic radiation and (2)applying an electrolytic solution to the exposed layer, said solutioncontaining a solute that is capable of being reduced to form a visibleimage at cathode sites in photoexposed areas of said layer by means ofsaid spontaneously induced voltage.

5. An electrophotographic process employing a photosensitiveautoelectrolytic element, which element comprises a photosensitive layerconsisting essentially of a physical mixture of photoconductiveinsulating particles and metal particles dispersed in an insulatingfilm, there being 3 to 10 parts by weight of photoconductive insulatingparticles for each part by Weight of metal particles, and about 3 toabout 10 parts by weight of photoconductive insulating material for eachpart of film-forming vehicle, said layer having the capability, afterexposure to actinic radiation, of spontaneously inducing voltage in anelectrolyte suflicient to cause electrolytic reduction at cathode sitesin photoexposed areas of said layer, said process comprising: (1)exposing selected areas on said photosensitive layer to actinicradiation and References Cited UNITED STATES PATENTS 3,380,823 4/1968Gold 96-27 3,392,018 7/1968 Metcalfe et a1. 96-1 3,152,903 10/1964Shepard et a1. 9664 GEORGE F. LESMES, Primary Examiner 15 I. C. COOPER,III, Assistant Examiner US. Cl. X.R.

